We hypothesize that there exists critical threshold levels of repopulation with anti-HIV gene-marked cells above which protection of the immune system from HIV will be successful, and below which, HIV-1 infection will lead to immune system failure. The landmark genetic and population studies showing protection from HIV- 1 in CCR5?32 individuals coupled with the remarkable case study demonstrating cure by allogeneic transplantation of CCR5?32 hematopoietic stem/progenitor cells (HSPC) has led to numerous transplantation studies using cells engineered to knockdown CCR5 expression. However, determining the critical levels of repopulation after transplant necessary to cure or provide life-long suppression of HIV-1 has not been addressed. Given the expected low levels of gene-marking by anti-HIV-1 gene engineered cells in clinical studies, it is critical to understand the parameters for repopulation at which a minimum level of gene engineered cells protect the individual from HIV-1 infection and restore normal immune function. Establishing and understanding the immune parameters at this threshold level is critical for determining experimental conditions for transplant such that repopulation with gene engineered cells is therapeutically effective in preventing HIV-1 from killing the majority of unprotected cells. Testing the above hypothesis is impossible in humans and impractical in non-human primates, so we propose to extend our understanding of repopulation and protection from HIV-1 to a humanized small animal model system. The BLT mouse system is ideal for studying HSPC transplant since unlike other humanized mouse models, the transplanted human HSPC differentiate in the context of a human thymus allowing normal human T-cell differentiation and resulting in robust reconstitution of T-cells (as well as macrophages and Bcells) in peripheral blood and in all the major tissue sites of HIV-1 replication. We recently published in a rhesus macaque transplant model highly analogous to human transplant that thousands of HSPC clones contribute in temporal waves and by diverse lineage potential to repopulation. Like the simian model, we show that repopulation in BLT mice is polyclonal. Transplant can be experimentally manipulated to model different parameters of repopulation with anti-HIV gene engineered cells. To analyze our observations and measurements of T-cells, we will develop mathematical models that capture the key biological processes. Such models will not only help define threshold levels of transplantation, but will also allow one to make systematic predictions under different transplant protocols and experimental conditions. For example, the effects of gene therapeutic reagent and transplantation dosage can be directly investigated. Moreover, by including known equations describing the kinetics of HIV-1 infection, viral loads, the influence of different combinations of antiviral therapies, and the probabilities of extinction and total HIV-1 clearance in organisms can also be explored.